1. The Field of the Invention
The present invention relates to methods and apparatus for sterilising the interior of a chamber using either a two component or a multi-component vapour, one component of which will be water.
2. The Relevant Technology and Summary of the Invention
There are numerous applications for sterilising the interior of a chamber including its contents in the pharmaceutical, biotechnology, and food industries, as well as the medical world. A number of compounds have been used as sterilising agents some of which are only partially effective and some of which have serious side effects because they are toxic, corrosive, or can cause other environmental damage. Formaldehyde has long been used as a cheap and quite effective sterilising agent but doubts over its safety and environmental persistence may prevent continued use. Hydrogen peroxide is a simple and cheap compound with good sterilising properties. Its major advantage is that it can be decomposed to water and oxygen, which are totally harmless products. In the vapour phase, hydrogen peroxide can be used to treat work areas of size from safety cabinets to clean rooms. In all gas phase sterilisation, deep layers of contamination will not be effected and good cleaning procedures are necessary as a preliminary to gas phase sterilisation.
Hydrogen peroxide gas sterilisation and decontamination systems have been designed in order to avoid condensation, and as such both flow through and recirculating systems have been so organised as to keep the vapour concentrations, especially of water, below the dew point. Examples of such systems are described in European Patent EP 0486623B1, UK Patent 2217619B, WO89/06140 and UK Patent 2308066A.
More recent work has shown that for rapid surface sterilisation and decontamination in rooms and smaller chambers, or isolators, condensation of a mixture of vapours of a gaseous decontaminant such as hydrogen peroxide and water is essential. It is now believed that gaseous surface sterilisation using hydrogen peroxide is a condensation process so it would seem sensible to examine the process, to see how it may be optimised to take advantage of the condensation process. This knowledge may then be applied not only the sterilisation process using hydrogen peroxide gas but also other mixtures of sterilising gases that rely on condensation for their activity.
In the apparatus described in the European Patent EP 0 486 623 B1 the air/gas mixture is circulated through the sealed chamber to be sterilised and then through the apparatus to produce and control the gas mixture. The gas returning to the apparatus is cleansed of any hydrogen peroxide gas and also dried before more water vapour and hydrogen peroxide gas are added. This cleansing and drying process is likely to be wasteful, as the vapours removed from the circulating gas must be replaced so that condensation may occur in the sealed chamber. The only reason for the removal of these vapours would be if the concentration of the hydrogen peroxide gas had been reduced because of decomposition.
It is now well understood that vapour phase decomposition does not occur at room temperatures, such homogenous decomposition only happens at elevated temperatures as reported in Hydrogen Peroxide, WALTER C. SCHUMB, CHARLES N. SATTERFIELD, RALPH L. WENTWORTH, AMERICAN CHEMICAL. Society, Monograph Series, Catalog Card Number 55-7807, Chapter 8. Decomposition does however happen on surfaces, which are catalytic, but this appears to be very small amounts. To date no observer has seen a measurable increase in oxygen concentration, and the measured hydrogen peroxide gas concentrations conform very closely to the saturated vapour pressures of the original aqueous solution that is evaporated into the air stream. All of the indications are therefore that the amount of vapour phase decomposition of hydrogen peroxide is very small.
Since this sterilisation process relies on condensation of the hydrogen peroxide vapour then the most critical parameter is the rate at which this condensation may be achieved. The amount of hydrogen peroxide vapour available for condensation within the sealed enclosure will depend on the vapour concentration delivered to the chamber and the concentration leaving the chamber. The difference between these two amounts will be the quantity of hydrogen peroxide that is available to form a film of condensation.
The maximum concentration of vapour that can be delivered to the chamber depends on the temperature of the gas stream entering the chamber, the concentration of the aqueous sterilising solution being evaporated into the gas stream and the total water content of the gas. The carrier gas, normally air, that is used to transport the sterilising vapours through the total system will never be perfectly dry, even after passing through the drying system. This additional water in the carrier gas will dilute the hydrogen peroxide to a small extent and this additional water will reduce the amount of hydrogen peroxide that may be carried by the gas. The concentration of the vapour leaving the sealed chamber, once stable conditions have been reached, will be determined by the saturated vapour pressure for the conditions at the exit of the sealed chamber. Thus, if it is assumed that only insignificant amounts of decomposition occur, then the rate of condensation will depend on the concentration of the gases delivered to the chamber and the temperature of the gases leaving the chamber.
In general there are two factors that are important when considering a gaseous surface sterilisation process. The first and most important is to be sure that the process has been successful and the second is to achieve sterilisation in the minimum possible time. The most common technique for assuring sterility is to develop a cycle and to test the performance with biological indicators. This cycle development will include optimisation of each phase of the sterilisation cycle. This is a complex issue as there are many parameters to be considered during the optimisation process apart from the obvious considerations of gas concentration and flow. Some of the less obvious ones are the starting value of the relative humidity, the moisture content of any microorganism, the rate of condensation, and the length of time it may take for the condensate to kill any microorganism. The technique used for the removal of the sterilant gas at the end of the cycle will also have marked influence on the total cycle time.
The optimised cycle then becomes fixed using the same physical parameters such as flow rates, times etc., but does not take into account any external factors that may change, e.g., the external temperature which will have an influence on the effectiveness of the cycle.
The problem with this fixed technique is that if some external influences change which have not been taken into consideration during the cycle development then a cycle, although properly developed, may become unsuccessful. The best method to overcome this difficulty is to measure those parameters that actually cause the sterilisation and use these measurements to control the cycle, rather than to use a set of predetermined factors to run identical cycles. The technique of using the measurements to control the cycle will lead to changes in the details of the cycle to counteract any changes in the circumstances surrounding the process.
This procedure also has the advantage of ensuring the minimum reliable cycle time, since the process will progress to a point where it is effective and no further. It is not necessary to add large safety margins to the cycle to ensure that it is effective, as the point at which it is effective is known from the measurements.
The objects of the present invention are to control the sterilisation cycle using sensors, and to provide a recirculating system that does not require the steps of removing water vapour and sterilising gas mixtures during the critical sterilisation phase of the cycle.
This invention provides a method of sterilising a sealable enclosure comprising the steps of initially reducing the relative humidity in the enclosure to about 30 to 40%, circulating a carrier gas to the enclosure, raising the temperature of the circulating gas above ambient, supplying a sterilant vapour or vapours to the circulating carrier gas sufficient to saturate substantially the gas whereby on cooling in the enclosure, a condensate of the sterilant vapour is formed on surfaces in the enclosure, distributing the gas/vapour throughout the enclosure to ensure that the condensate is formed on all surfaces in the enclosure, measuring the amount of condensate formed on a surface of the enclosure and continuing to circulate the gas/vapour until a required amount of condensate has formed in the enclosure terminating supply of sterilant vapour to the gas whilst continuing to circulate the saturated gas/vapour to maintain the condensate on the surface for a predetermined period of time and finally extracting the sterilant vapour from the carrier gas whilst continuing to circulate the carrier gas through the enclosure to extract the condensate from the enclosure.
Preferably the sterilant vapour is extracted from the carrier gas by breaking down the vapour into disposable constituents.
It is also preferred that the sterilant vapour is hydrogen peroxide and water vapour. In this case the hydrogen peroxide extracted from the chamber with the circulating gas is subjected to catalytic action to break the hydrogen peroxide down into water vapour and oxygen, the former being extracted from the gas before the gas is recirculated through the enclosure.
The initial step of reducing the relative humidity in the enclosure may be carried out by circulating the carrier gas through the chamber and extracting water vapour from the circulating gas outside the chamber.
The relative humidity in the enclosure may be reduced initially to about 35%. In addition, the enclosure may be held at said reduced relative humidity for a period of time according to the size of enclosure and flow rate of gas to ensure dryness of said surfaces in the enclosure.
The invention also provides a method of sterilising a sealable enclosure comprising the steps of circulating a carrier gas through the enclosure and through a flow path having an inlet to the enclosure and an outlet from the enclosure, the flow path having parallel branches in one of which any sterilant in the gas flow is rendered suitable for disposal and any water vapour content in the gas is reduced and the other of which the carrier gas is heated and sterilant is added to the gas, the method further comprising the steps of initially circulating said carrier gas through said one branch, monitoring the moisture content of the gas in the enclosure and terminating flow of carrier gas through said one branch when the relative humidity in the enclosure has been reduced to a predetermined level such that the surfaces of the enclosure are relatively dry, initiating flow of the carrier gas through said other branch and adding a sterilant vapour or vapours to the gas passing through the other branch until condensation of the sterilant takes place in the enclosure, terminating supply of sterilant to the carrier gas, continuing to circulate the carrier gas substantially saturated with sterilant vapour for a predetermined time to ensure sterilisation of the enclosure terminating flow through said other branch and redirecting the flow of carrier gas through said one branch to extract the sterilant from the gas enclosure to render the sterilant suitable for disposal and to reduce the relative humidity of the carrier gas.
Entry to one branch is closed and entry to the other branch may be opened and vice versa to provide flow through one or other of the branches. For example, valve means may permit flow into one branch and not the other and vice versa.
Alternatively, pump means may be provided in said parallel branches and are used to cause gas flow along one or other of the parallel branches in the flow path.
The invention also provides an apparatus for sterilising a sealable enclosure comprising a circuit for flow of carrier gases in which the enclosure to be sterilised may be connected to form a closed circuit with the enclosure, means for causing a carrier gas to circulate through the circuit including the enclosure, means to remove water vapour from the circulating gas to dry the enclosure and surfaces thereof, means to raise the temperature of the circulating gas above ambient, means to supply a sterilant vapour or vapours to the circulating gas to form, on cooling in the enclosure, a condensate on the surfaces in the enclosure, means to distribute the gas/vapours throughout the enclosure to ensure that the condensate is formed on all surfaces in the enclosure, means to measure the amount of condensate formed on a surface of the enclosure and to terminate the supply of sterilant vapour to the carrier gas when the required level of condensate has formed, the circulation of carrier gas continuing to extract condensate from the enclosure.
The invention further provides an apparatus for sterilising a sealable enclosure comprising a circuit for flow of a gas or gasses, the circuit having means to receive and connect an enclosure to be sterilised in the circuit to form a closed circuit therewith, means to circulate gas through the circuit and enclosure, and having parallel branches in the circuit one of which contains means to deactivate a sterilant to be added to the carrier gas flowing through the circuit and means to dehumidify the gas and the other of which branches contains means to heat the gas and means to supply a sterilant vapour or vapours to the gas, the apparatus further comprising control means for determining through which of the parallel branches the gas flows, the control means including means to determine the relative humidity of the gas exiting the enclosure to maintain said one branch passage open until the relative humidity falls below a predetermined level and then to close that branch and to open the other branch and means to measure condensation in the enclosure to close said other branch and to open said one branch when the required amount of condensation has built up in the enclosure.
It has been found that in aqueous solutions of hydrogen peroxide very fast kill rates are achieved even at 10% hydrogen peroxide concentrations with even faster kills at 20% solution. Since we believe that gaseous surface sterilisation is a micro condensation process, then it may be considered to be analogous to the work described in the publication by THE STERILISING EFFECT AGAINST BACILLUS SUBTILIS SPORES OF HYDROGEN PEROXIDE AT DIFFERENT TEMPERATURES AND CONCENTRATIONS; P. Swartling and B. LINDGREN J. DAIRY RES. (1968), 35,423. This gives a good guide as to the expected results that may be achieved with a gaseous condensation process.
This also suggests that should some small amount of decomposition occur because of surface catalysation of the gas then kills would still be achieved. In reality such decomposition appears to be very small indeed as indicated by the gas concentration data.